1. Field of the Invention
The invention is directed to an arrangement for level conversion of high-frequency low-voltage signals that are to be transmitted between systems of different low-voltage technology fields.
2. Description of the Related Art
The transmission of high-frequency low-voltage signals is increasing in significance, whether in the transmission of data signals or in the transmission of clock signals. The low-voltage signals are generated at the emitting side within the boundary conditions and parameters of a specific technology field. Attempts have always been made to employ the same technology at the receiver side in order to avoid losses and to particularly reduce attenuations.
Applications have been identified in which changing technology fields is advantageous. In such instances, however, it is necessary to adapt the high-frequency low-voltage signals to be transmitted that have been generated within the boundary conditions and parameters of one technology system to the boundary conditions and parameters of the other technology system. This is even required when the same supply voltage conditions are employed, for instance Vcc=3.3 V to ground potential.
A typical case of a level conversion is the transmission of clock signals from a low-voltage pseudo-ECL system to a low-voltage CMOS system and vice versa.
The problem underlying the invention is explained in greater detail on the basis of FIG. 2 and a specific application, namely the clock supply of high bit rate switching network structures in a narrowband switching system such as EWSD with a PLL system comprising a quartz oscillator, as explained, for example, in German Patent Application No. 199 43 172.8 filed Sep. 9, 1999.
FIG. 2 shows a function unit with which the switching network clock CLK2 at 2.048 KHz and an appertaining frame marking bit signal or, respectively, frame clock signal FMB2 at 8 KHz supplied from a preceding function unit (not shown) is converted into the required high-frequency clock signals. This function unit contains a middle, analog or, respectively, discrete part 21 with a 184 MHz sine oscillator in an LC structure in the form of a VCO 6. A low-voltage CMOS part 22 and a low-voltage pseudo-ECL part 23 are also provided, where the term xe2x80x9clow-voltagexe2x80x9d or xe2x80x9clow-tensionxe2x80x9d is respectively indicated by the abbreviation LV.
At its input side, the low-voltage CMOS part 22 receives the clock signal CLK2 and the frame clock signal FMB2. At its output side, the low-voltage pseudo-ECL part 23 outputs a high-frequency clock signal CLK92, preferably at 92.16 MHz, and a frame marking clock signal or, respectively, frame clock signal FMB92, likewise at 8 KHz, that is decoupled from the frame clock signal FMB2 of the input side. The supplied clock signal CLK2 is supplied to a phase detector 8 (PD) in the low-voltage CMOS part 22, the phase detector being constructed in the form of an exclusive-OR element.
The output signal of the low-voltage CMOS part 22 enters via a low-pass filter 9 (loop filter) with a limit frequency of 200 KHz into a VCO 6 that is discretely constructed. The output signal of the VCO 6 at 184 MHz is an input signal of a comparator 73 of a converter 7 in the low-voltage pseudo-ECL part 23. The comparator 73 compares to an internal reference signal 74 and outputs its output signal to two dividers 71 and 72. Here, the divider 71 is a 1:2 divider that outputs a signal with the frequency 92.16 MHz at its output side as clock signal CLK92. The other divider 72 is a 1:6 divider that feeds back an output signal at 30.72 MHz as feedback signal via a feedback loop 13. In the analog part 21, the feedback loop 13 contains a level converter 14 for the conversion from the low-voltage pseudo-ECL level onto the low-voltage CMOS level and further contains a step-down device 15 in the low-voltage CMOS part 22 that is fashioned as a 1:15 divider in order to again obtain a signal with the frequency 2.048 KHz. This signal is resupplied to the phase detector 8 as a feedback signal.
The low-voltage CMOS divider 22 also contains a synchronizer 18 that receives the frame clock signal at 8 KHz at its input side and that is synchronized by the fedback signal divided down by the step-down device 15. The output signal of the synchronizer 18, which is already decoupled in phase position here from the phase of the frame clock signal FMB2 of the input side, is supplied to a phase element 16 in the low-voltage pseudo-ECL part 23 via a level converter 17 in the analog part 21 for conversion from the low-voltage CMOS level onto the low-voltage pseudo-ECL level, the phase element 16xe2x80x94for phase drivexe2x80x94receiving the output signal of the divider 72 or of an identical divider as well as the output signal of the divider 71, i.e., the high-frequency clock signal CLK92, and, using these signals and two ultra-fast flip-flops in the phase element 16, adapting the phase position of the output frame clock signal FMB92 with 8 KHz to the output clock signal CLK92, first roughly and then finely.
What the division of the components onto the CMOS part 22 and the pseudo-ECL part 23 effects is that time-critical functions are realized in the low-voltage pseudo-ECL part 23 and gate-intensive functions are realized in the low-voltage CMOS part 22. The functions in the low-voltage CMOS part 22 are thereby expediently realized with standardized, electrically programmable logic fields (FPGA), whereas the functions in the low-voltage pseudo-ECL part 23 are realized with the assistance of ultra-fast, discrete components. The connection of the interfaces of said two parts 22 and 23 occurs in the middle part 21 via exclusively analog or, respectively, passive components, as a result of which the immunity of the overall system to noise influences is enhanced. The required power supply of the individual parts 21, 22 and 23 is expediently island-like/isolated, i.e., respectively separately implemented, in order to suppress noise infeeds among one another and from clock-alien regions of the various parts. Expediently, the level converters 14 and 17 should prevent a source for jitter or for noise emissions from arising.
The initially cited problems arise in the implementation of such an arrangement. This is especially true when each of the parts 22 and 23 is optimally designed in view of the payload signals to be processed, particularly when using commercially available components.
The module MC100LVEL38 of Motorola, a xe2x80x9clow skew clock generation chipxe2x80x9d, was proposed in the low-voltage pseudo-ECL part 23 in one implementation. This module is optimally suited for the conversion of high-frequency sine signals (from the VCO 6) into the required logic level in the low-voltage pseudo-ECL part 22. As a result of an internal 1:6 frequency divider, use in high-frequency PLL applications to far above 100 MHZ is especially beneficial since a feedback signal can be output to the low-voltage CMOS part 22, namely its step-down device 15, without intermediate amplifiers or dividers. This can be realized by standardized, electrically programmable logic fields FPGA that have a relatively high input capacitance of 15 through 20 pF. The phase element 16 can in turn be implemented with the module MC100LVEL51 of Motorola, an xe2x80x9cultra high speed differential clock D-flip flopxe2x80x9d. As a result of its negligible gray switching area (clock/data-timing ts+thxe2x89xa6100 ps), this module is optimally suited for the sampling function in the phase-in in the low-voltage pseudo-ECL region 23 of the high-frequency signal.
The following technology families have been taken into consideration in the low-voltage CMOS part 22 in the realization:
Specific high-speed connections can be realized in the TGC family with the I/O cell pair LVDS_O and PECL_I that work with a quasi low-voltage pseudo-level matched to one another.
In the XC 4000 family, specific LVCMOS_I input cells that are distinguished by their higher sensitivity for HF applications are available in addition to the standard TTL or, respectively, LVTTL cells. Only the standard TTL input type comes into consideration for HF PLL applications in the FLEX 6000 family.
Very different parameters derive in such an implementation, even when the same supply voltage system is employed, although constructed island-like as mentioned above. The threshold relationships are of significance in the present case.
A brief overview of the threshold relationships of the various input types is presented in the following table.
An output voltage that is higher than 2.0 Vss is desired for a universal usability in the level conversion thus deriving from the low-voltage pseudo-ECL domain (at the left in FIG. 3) into the low-voltage CMOS domain (at the right in FIG. 3).
FIG. 3 thus shows a level diagram of the conditions that must be adhered to for the level conversion from the low-voltage pseudo-ECL domain to the low-voltage CMOS domain.
The low-voltage pseudo-ECL level of 0.8 through 0.9 Vss should therefore be amplified by about a factor 3 (corresponding to 10 dB) to a level range from 2.4 to 2.7 Vss, whereas the dc voltage level should be lowered from approximately 2.0 V to 1.4 V, the nominal TTL threshold. The desired attenuation of the dc voltage level therefore amounts to approximately xe2x88x923.3 dB (a=0.68), where the level itself should not be co-attenuated. Since the low-voltage level in the low-voltage pseudo-ECL domain is linked to the supply voltage (Vcc=3.3V), the stability and the filtering of the supply voltage are very important for use in low-jitter PLL circuits.
The different parameter conditions in the opposite direction, i.e., from the low-voltage CMOS domain (at the left in FIG. 4) to the low-voltage pseudo-ECL domain (at the right in FIG. 4) are shown in greater detail in FIG. 4. This involves the most optimum adaptation of the various levels by an attenuation and level shifting. These givens must be taken into consideration due to the presence of high-frequency signals. In particular, the dependability of the phase-in can be improved when the edge steepness can be increased.
It is thus an object of the present invention to specify arrangements for level conversion that allow a simple yet broadband realization. This object is achieved by an arrangement for level conversion of high-frequency low-voltage signals that are transmitted between systems of a first low-voltage technology field circuit and a second low-voltage technology field circuit, comprising:
a high-frequency transformer comprising a primary and secondary winding;
a converter circuit comprising a first side and a second side;
the primary winding of the transformer being connected to an output signal of a drive stage at an output side of the first technology field circuit;
the secondary winding of the transformer being connected to the converter circuit on the first side of the converter circuit,
the converter circuit being connected via an output signal of the converter circuit to a high input-side load capacitor of the second technology field circuit on the second side of the converter circuit;
the converter circuit comprising:
a dc coupling of the transformer; and
a series resistor and a parallel resistor for setting a dc level of the output signal of the converter circuit.
The converter circuit may further comprise a coupling capacitor connected in parallel to the series resistor, and may further comprise a further series resistor for an emitter current supply of an output transistor of a driver stage of the first technology field circuit.
The object is also achieved by arrangement for level conversion of high-frequency low-voltage signals that are transmitted between systems of a first low-voltage technology field circuit and a second low-voltage technology field circuit, comprising an RC attenuation element tuned in high-frequency fashion between: a) an output of an output driver of the second low-voltage technology field circuit, and b) a load capacitor as well as a load resistor of an input transistor circuit of an input transistor of the first low-voltage technology field circuit.
The RC attenuation element may also comprise a series resistor and a shunt resistor for setting a dc level as well as an overall current.
The first low-voltage technology field circuit may be formed by pseudo-ECL technology and the second low-voltage technology field circuit may be formed by CMOS technology.
The invention can be particularly employed in an arrangement for clock supply of high bit rate switching network structures.